1. Field of the Invention
The present invention is directed to a control system for reducing distortion produced by electrical circuits and in particular to a control system that uses an improved pilot detection technique.
2. Description of the Related Art
Electrical circuits in general often add undesired distortion to an input signal, creating an output signal comprising distortion components and the input signal component. The distortion includes any undesired signals added to or affecting adversely the input signal. There is therefore a need to devise techniques that can eliminate substantially or reduce significantly the distortion produced by electrical circuits. A well known technique uses a control system and a pilot signal. The pilot signal is an electrical signal comprising at least one frequency component spectrally located near the frequency band of operation of the electrical circuit. A more complete description of the pilot signal is shown in FIG. 1.
Referring to FIG. 1, the frequency response of the electrical circuit is shown including the location of the pilot signal. The pilot signal can be near the lower edge of the operating band (e.g., pilot 1) or located near the upper edge of the band of operation (e.g., pilot 2). The pilot is positioned a spectral distance of .DELTA.f from an edge of the band of operation whose center frequency is f.sub.0. The pilot signal can also be located somewhere within the band of operation of the electrical circuit. It is readily obvious that the electrical characteristics (e.g., amplitude, phase response, spectral content) of the pilot signal are known. It should be noted that although the pilot signal is shown as a single spectral component of a certain amplitude, the pilot signal can comprise a plurality of spectral components having various amplitudes.
The control system reduces distortion produced by the electrical circuit by applying the pilot signal to the electrical circuit and making adjustments based on information obtained from the applied pilot signal. FIG. 2 discloses the control system and its use of information obtained from the pilot signal to reduce distortion produced by electrical circuit 108. Electrical circuit 108 can be any electrical circuit having at least one input and one output. Say, for example electrical circuit 108 can be a Radio Frequency (RF) linear amplifier, or a power amplifier.
Still referring to FIG. 2, an input signal is applied to splitter 102. Splitter 102 replicates the input signal on paths 128 and 130. Splitter 102 is part of a feed forward loop referred to as loop # 1, which in addition to splitter 102, comprises Gain & Phase circuit 104, coupler 132, electrical circuit 108, delay circuit 106 and coupler 116. The input signal on path 130 is applied to Gain & Phase circuit 104. The output of Gain & Phase circuit 104 and the pilot signal are applied to coupler 132. Typically, the amplitude of the pilot signal is much less (e.g., 30 dB less) than the amplitude of the input signal so as not to interfere with the operation of electrical circuit 108. The output of coupler 132 is applied to electrical circuit 108 whose output comprises the input signal, the pilot signal and distortion signals produced by electrical circuit 108. A portion of the output of electrical circuit 108 is obtained from coupler 114 and is combined with a delayed version of the input signal (signal on path 127) at coupler 116 via connecting path 115. The signal on path 127 has experienced sufficient delay provided by delay circuit 106 so that such signal experiences the same delay as the input signal appearing at coupler 116 via path 115.
Gain & Phase circuit 104 is controlled via control path 134 with at least one control signal to adjust the gain and phase of the input signal such that the input signal appearing at coupler 116 via path 115 is substantially the inverse (equal in amplitude but 180.degree. out of phase) of the delayed input signal at coupler 116 via path 127. The control signal appearing on control path 134 of Gain & Phase circuit 104 is derived from the signal at point A in a well known manner such as the use of detection circuits. The detection circuits detect well known electrical signal characteristics such as amplitude, phase, and frequency of the signal. Therefore, the input signals applied to coupler 116 substantially cancel each other leaving at point A the pilot signal and distortion signals produced by electrical circuit 108. Loop # 1 is thus a feed forward loop which serves to isolate at point A the pilot signal and distortion signals produced by electrical circuit 108.
The signals appearing at point A (pilot signal and distortion signals) are fed to Gain & Phase circuit 110 whose output is fed to amplifier 112 whose output is applied to coupler 120 via path 126. A portion of the output signals (input signal, pilot signal and distortion signals) of electrical circuit 108 is fed to delay circuit 118 whose output is fed to coupler 120 via path 124. Delay circuit 118 is designed such that signals from the output of electrical circuit 108 applied to coupler 120 via path 124 experience substantially the same delay as the signals from the output of electrical circuit 108 applied to coupler 120 via path 126.
Because the frequency, amplitude and other electrical characteristics of the pilot signal are known, pilot detect circuit 140 can use circuits such as a mixer connected to a log detector (or other well known detection circuits) to detect the pilot signal or a portion of the pilot signal via coupler 138. The pilot signal is used to obtain information about the distortion produced by electrical circuit 108. The information is obtained by detecting well known electrical signal characteristics of the pilot signal. In particular, the characteristics (e.g., amplitude, spectral content, phase response) of the pilot signal are known and thus when pilot detect circuit 140 detects the pilot signal, some or all of the known characteristics may have been altered due to the distortion effects of electrical circuit 108. Detection circuit 140 will detect such distortion effects and use this information to generate control signals onto path 136 to cause Gain & Phase circuit 110 to modify the pilot signal at point A such that the pilot signal at coupler 120 via path 126 is substantially the inverse (equal in amplitude but 180.degree. out of phase) of the pilot signal at coupler 120 via path 126. The corresponding pilot signals and the distortion signals at coupler 120 cancel substantially each other at coupler 120 leaving the input signal (or an amplified version of the input signal) at the output of the control system. Therefore, loop # 2, which comprises coupler 114, coupler 116, Gain & Phase circuit 136, amplifier 112, coupler 120 and delay circuit 118 is a feed forward loop which uses the information obtained from the pilot signal to cancel substantially the distortion produced by electrical circuit 108.
In an actual circuit, there is rarely an absolute cancellation of the distortion and the pilot signals. Thus, the output signal of the control system of FIG. 2 still contains some relatively minute amount of distortion. The residual distortion, which exists in the feed forward loop (loop #2), is also a result of dynamic distortion characteristics exhibited by many electrical circuits such as electrical circuit 108. Amplifier 112 also adds distortion to the output signal. It is therefore desirable to detect the pilot signal and obtain information from the pilot signal to cancel the residual distortion.
The output of the control system contains the pilot signal, albeit very small in amplitude compared to the output signal, that can be used to cancel the residual distortion from the feed forward loop (loop # 2). The amplitude of the pilot signal is typically relatively small compared to the output signal because of the cancellation effects of the feed forward loop #2 as described above and the relative amplitude (also described above) of the pilot signal with respect to the amplitude of the input signal at the input of electrical circuit 108. Thus, it becomes very difficult to detect the pilot signal at the output of the control system.
One way of detecting the pilot signal is to use a filter with a very sharp and narrow spectral bandwidth. The use of a filter to detect the pilot signal is a very constricting design in that any change or deviation in the frequency and/or amplitude of the pilot signal would require the design of another filter. Different filters have to be used with different pilot signals making the design and use of a control system with such a pilot detect design limiting and somewhat impractical. Also, the requirement to use different filters based on the selected pilot signal tends to complicate the design of the control system and increase the cost of such a system.
Therefore, in order to improve the distortion cancellation effects of the control system of FIG. 2, there exists a need to have a flexible design for detecting the pilot signal without significantly adding to the complexity and cost of the control system.